This invention relates to an integrated circuit assembly, and more particularly to the structure by which an integrated circuit is supported from a base board.
In one common architecture, a microelectronic circuit is prepared as a chip or a die, termed herein an integrated circuit. The integrated circuit is fragile and is typically supported on and electrically interconnected to a base board such as a printed wiring board (PWB). A cover is placed over the integrated circuit to protect it mechanically and against damage during handling, completing the integrated circuit assembly. Large numbers of integrated circuits may be supported on a single base board and under a single cover, forming a multi-chip module (MCM).
The structural and electrical interconnection between the integrated circuit and the base board may be made in a variety of ways. In one, electrical leads extend directly from the integrated circuit to interconnects on the printed wiring board. In another, solder bumps are used to join the components together. During fabrication of the integrated circuit assembly, the solder bumps are contacted to corresponding pads on the facing structure and heated to bond the components together. The solder bumps provide not only the structural and electrical interconnect, but also a heat flow path for removing heat from the integrated circuit.
This approach is operable, but the structure is subject to thermal cycling damage during fabrication processing or during service. The integrated circuit and the base board have quite different coefficients of thermal expansion. As the temperature changes, thermal strains and thermal stresses are generated within the assembly. If the thermal strains and thermal stresses become excessive, the structure may structurally and/or electronically fail at the solder bumps or elsewhere.
To overcome this problem, the solder bumps may be positioned laterally outwardly from the integrated circuit, so that the thermal strains and thermal stresses are lessened. This solution results in a larger size and weight of the assembly, as well as a lengthening of the heat flow path from the integrated circuits to the heat sink.
There is a need for an improved approach to the joining of an integrated circuit to its support to form an integrated circuit assembly. The present invention fulfills this need, and further provides related advantages.
The present invention provides an integrated circuit assembly that achieves electrical and mechanical integrity while reducing the susceptibility of the structure to thermal cycling damage. The integrated circuit assembly may be protected against corrosion damage without being hermetically sealed, thus reducing fabrication and rework difficulties. The structure of the integrated circuit itself is not altered, so that it may be prepared and optimized for its required functionality.
In accordance with the invention, an integrated circuit assembly comprises a base board, an interposer, and an array of solder balls electrically and structurally interconnecting the interposer and the base board. The interposer comprises a backbone layer having a backbone stiffness, and at least one compliant layer overlying and contacting the backbone layer. The solder balls are preferably formed of a lead-tin-base solder having a melting temperature of greater than about 210xc2x0 C. The compliant layer has a compliant-layer stiffness of less than the backbone stiffness. An integrated circuit is supported on the interposer, and a cover optionally overlies and protects the integrated circuit. The cover may be a shell or a flowable encapsulant. The cover may have an opening therethrough. In most cases, an electrical interconnect extends from the integrated circuit to at least one of the solder balls, which in turn are in electrical communication with electrical conductors on the base board.
More preferably, the interposer comprises the backbone layer having the backbone stiffness, a first compliant layer affixed to the backbone layer between the backbone layer and the integrated circuit, the first compliant layer having a first-layer stiffness of less than the backbone stiffness, and a second compliant layer affixed to the backbone layer between the backbone layer and the solder balls, the second compliant layer having a second-layer stiffness of less than the backbone stiffness. The first-layer stiffness and the second-layer stiffness are preferably, but not necessarily, the same. The backbone may be made of a fiber-reinforced polymeric material, and each compliant layer is made of a compliant polymeric material which is not fiber reinforced.
The use of the compliant layer(s) reduces thermal strains and stresses generated within the structure during thermal cycling, thereby reducing the incidence of thermal-cycling failure. The life of the structure is thereby extended. This approach permits the solder balls to be vertically registered below the integrated circuit, so that the heat flow path through the solder balls is short and of low thermal impedance. Heat removal from the integrated circuit is thereby facilitated.
In one embodiment, the integrated circuit includes integrated-circuit bonding pads thereon. A protective coating overlies the integrated-circuit bonding pads. The protective coating preferably comprises a layered metal selected from the group consisting of copper-nickel/gold and titanium-tungsten/gold.
The use of the protective coating protects the integrated-circuit bonding pads against moisture damage and other corrosive effects, both in fabrication operations and in service. The remainder of the integrated circuit is ordinarily protected against moisture and other corrosion by protective layers such as silicon nitride or silicon oxide. Together, the protective coating and the protective layers encapsulate and protect the integrated circuit and its bonding pads, so that the remainder of the package may be non-hermetic without concern for corrosion damage of the integrated circuit. The cover may be provided even in this case to protect the integrated circuit against mechanical damage.
The present approach is most usefully employed where there are multiple integrated circuits which are assembled to the single base board using the described approach to make a multi-chip module (MCM). The solder reflow operations associated with assembly and, where necessary, rework of the MCM are aided by using a temporary cover with an opening therethrough to permit pressure equalization during the reflow soldering process. If one integrated circuit fails during service, the integrated circuit may be replaced easily. This non-hermetically sealed MCM is fully reworkable.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.